Electric wires or cables having dielectric layers of polyolefins such as a polyethylene, cross-linked polyethylene and the like are in wide use. These polymers have in recent years been improved so that they are able to withstand high applied voltages. One important problem at present with these electric wires and cables is that they undergo a gradual degradation with time under continuously applied voltages, resulting in a reduction in their capability to withstand applied voltages. Because the service life of electric wires or cables is as long as thirty years, the initial thickness of the dielectric layers must be increased to compensate for the reduction in their ability to withstand applied voltages with the passage of time.
Moreover, recent urban developments have required increased power transmission capacity for electric wires or cables while keeping the volume thereof as small as possible. Therefore, a problem has developed in that electric wires or cables, having a dielectric layer consisting essentially of a polyolefin, require the ability to withstand increasing transmission voltage without increasing the thickness of the dielectric layer.
It is generally believed that the degradation referred to above results from the formation of "water trees" in a cross-linked polyolefin insulation.
Various approaches to overcome the above problem have been considered, including preventing the occurrence of the above degradation, whereby the decrease in capability to withstand applied voltages during a thirty year service life need not be taken into account, and the requirements of urban areas regarding high transmission capability and low volume can be met.
For example, improved cables or cable joints are described in Japanese Patent Publication No. 37668/79, Japanese Utility Model Publication No. 29008/74, Japanese Patent Publication No. 51180/81 and corresponding U.S. Pat. No. 4,092,688. In these cables or joints a polyethylene composition with an ethylene-vinyl acetate copolymer compounded therewith is used as an insulating material, whereby resistance against degradation has been improved to a certain extent.
Recently, however, it has been discovered that when cross-linked polyethylene cables, particularly those cables covered with an insulating material, are used at high temperatures (e.g., 90.degree. C.) for long periods of time, they are seriously degraded.